tommath/bn_mp_prime_fermat.c

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#include <tommath_private.h>
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#ifdef BN_MP_PRIME_FERMAT_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
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*/
/* performs one Fermat test.
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*
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* If "a" were prime then b**a == b (mod a) since the order of
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* the multiplicative sub-group would be phi(a) = a-1. That means
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* it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
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*
* Sets result to 1 if the congruence holds, or zero otherwise.
*/
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int mp_prime_fermat(const mp_int *a, const mp_int *b, int *result)
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{
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mp_int t;
int err;
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/* default to composite */
*result = MP_NO;
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/* ensure b > 1 */
if (mp_cmp_d(b, 1) != MP_GT) {
return MP_VAL;
}
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/* init t */
if ((err = mp_init(&t)) != MP_OKAY) {
return err;
}
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/* compute t = b**a mod a */
if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
goto LBL_T;
}
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/* is it equal to b? */
if (mp_cmp(&t, b) == MP_EQ) {
*result = MP_YES;
}
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err = MP_OKAY;
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LBL_T:
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mp_clear(&t);
return err;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */